Some companies have done it, some companies have not. Some are/were cast. Some are forged from aluminum. Some have been high end CNC machined from billet. It depends on the brand and the time of manufacturing. Can you narrow the field down a bit? Is there a specific brand or manufacturer that you are looking into?

All Ar receivers are milled....but as Dillinger reported...from what ?

Most so-called Mil-Spec uppers and lowers are from forgings for strength.

Cast uppers & lowers run from old sand castings to lost wax to state-of-the-art investment castings .

High end receivers have been made by CNC milling of Billet stock....a very expensive process due to the time requirred in the parts and the cost of materials.
This process was originally done by Rocky Mountain Arms on their high end PATRIOT 223 pistols and rifles.

Castings or forgings are the most economical to complete into receivers as the basic shapes and contours are already present.

Broach cutting of the mag well is most common with a 6 to 8 foot broach.
Some have been done on a wire EDM machine but that is much more expensive.

So basically, all are milled...the question is milled from what ?

Some manufacturers make them exactly to the specs of the Army's tech data pack...like Les Baer. Others make them to a looser tolerences or reverse engineer the parts..and they show it. You get what you pay for.

I believe it was DPMS who used to offer steel lowers. They were of stainless steel and could be had in raw stainless or a black coated stainless (unsure what the coating was). They were never offered as a complete rifle only as a stripped lower. Brownells used to carry them but this was about five years ago. I assume they are no longer made. The purpose was for building target and varmint rifles where weight didn't matter. I don't see the point functionally or structurally since the lower is basically nothing more than a platform for the upper and a carrier for the fire control group and magazine.